This application claims priority to German Patent Application 101 05 255.3, filed Feb. 6, 2001, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to a device for converting an input digital signal sequence having an input sampling rate into an output digital signal sequence having an output sampling rate which differs from the input sampling rate. Such a device is generally known as a resampler. The present invention also relates to a method of this type.
2. Related Art
Such a resampler is known from European Patent Application 0 665 546 A2, for example. With a resampler, the ratio of the input sampling rate to the output sampling rate must first be determined. In the publication indicated above, this is performed by a gate time measurement. In an interpolator, samples at the output sampling times predetermined by the output sampling rate are interpolated from the samples of the input signal sequence. The interpolator is controlled by the sampling rate ratio detected. Since the determination of the sampling rate ratio is subject to measurement inaccuracies, buffering is provided in a buffer memory, e.g., a FIFO, at the output of the interpolator in the case of down-sampling and at the input of the interpolator in the case of up-sampling. According to the European Patent Application 0 665 546 A2, the sampling rate ratio controlling the interpolator is regulated as a function of the filling level of the buffer memory.
Regulation of the sampling rate ratio (ratio) as a function of the filling level of the buffer memory as proposed in the European Patent Application 0 665 546 A2 has the disadvantage that when there is a change in the filling level of the buffer memory, the group propagation time of the digital signal is altered by the resampler. When used in mobile wireless technology, for example, major changes in filling level of the buffer memory of xc2x11, for example, i.e., a change by one memory unit, cannot be tolerated because they would lead to fluctuations in signal propagation time through the resampler. In the case of the filling level controller of the buffer memory proposed in European Patent Application 0 665 546 A2, deviations in clock rate ratio are detected relatively late, if there has already been a relatively great detuning of the ratio. This leads to major interpolation errors due to incorrect sampling times.
A resampler with phase estimation but without any observation intervals having a variable observation length is known from German Patent Application 101 02 166 A1, which was published later.
An object of the present invention is to provide a device (resampler) and a method (resampling method) for converting an input digital signal sequence having an input sampling rate into an output digital signal sequence having an output sampling rate which operates with a high precision and with which a short acquisition time is sufficient.
This object, and other objects of the present invention, is achieved by the features of claim 1 with regard to the device and by the features of claim 11 with regard to the method. The dependent claims contain advantageous embodiments of the device and the method.
The present invention is based on the finding that accuracy in control of the interpolator or in establishment of the sampling times of the output signal sequence can be increased considerably if the control is based not only on an estimate of the sampling rate ratio but also on an estimate of the phase angle with observation intervals of variable length. Through the phase-coherent control according to the present invention, a deviation in the sampling rate ratio is detected before the deviation becomes so great that it leads to an increase or decrease in the storage level in the buffer memory (FIFO). This prevents a significant change in group propagation time through the resampler, which is associated with a change in memory level, and increases the interpolation accuracy of the interpolator. At the start of operation, observation intervals may be kept relatively short to achieve a short acquisition time. Accuracy can be increased through a subsequent incremental increase in observation length.